1. Field of the Invention
The present invention is directed to an automatic battery switch which can be used in devices that require at least two power states. More particularly, the present invention is a device that switches from n to n+m cells, can function as a voltage regulator and can significantly prolong functional battery life.
2. Discussion of the Related Art
Many battery powered devices, such as cellular telephones, etc., draw varying amounts of power depending on their immediate function. For example, a cell phone monitoring for an incoming call powers only the receiver and draws a small amount of current. When it is transmitting during a call, a considerably larger current is needed to power the transmitter. Thus, if a cell phone requires a minimum power supply (battery voltage) of 7.0 V to work and six NiCd cells (1.2 V each) are used, the total voltage will be 7.2 V. However, under high current load, the voltage will sag to a lower level and the cell phone may cease to work. If eight cells are used, the low current voltage will be 8.times.1.2 V, which is 9.6 V, which may be too much voltage for some of the circuitry.
A typical solution to this problem, is to add a voltage regulator. However, this solution can have many drawbacks. For example, if the electronic device has a very low standby current of 2-3 .mu.A (i.e., to keep a memory alive), the regulator's additional 8-9 .mu.A adds significantly to the quiescent power consumption (8-9 .mu.A is about as low a quiescent power consumption as any commercially available linear regulator attains). Further, the linear regulator has a "drop-out" voltage, which is defined as the difference between the input and output voltages. This is typically 1.0-1.2 V for most current loads. For some batteries that have flat discharge curves (such as the LiMnO.sub.2 battery), this additional 1.2 V reduces the useful battery life by as much as 50%. As previously mentioned, if the cell phone or other electronic device requires a minimum of 7.0 V to work, the battery must now produce 7.0+1.2=8.2 V. Some batteries still retain half of their overall capacity at this new voltage cutoff point.
Thus, there is a need to provide a high speed automatic switch circuit which can switch between at least two power states and functions as a voltage regulator.